Many modern magnetic recording devices, such as disk drives, video tape recorders and tape data recorders, use multiple substantially parallel tracks on magnetic media. Data is encoded onto the magnetic media in the form of magnetic transitions induced in the media by magnetizing selected regions using electromagnetic write heads. Because of the nature of the media, these transitions remain on the media and induce a playback signal in electromagnetic read heads subsequently passed over the track in a playback mode.
Clearly, following the correct track is essential to correctly retrieve the desired data. However, there is great pressure to increase the densities of tracks of data on the magnetic media in order to increase the storage capacity of the media. This density increase leads to ever-narrower tracks. It follows, therefore, that tracking is complicated by such factors as mechanical vibration of the actuator, disk or spindle assembly, thermal track shifts and other mechanical sources of mistracking. These effects create track shifts that are largely independent of track-width, and so they present a far greater problem as tracks become narrower. Hence, the pressure to increase track density leads to increased problems in providing accurate tracking.
A number of approaches have been used in the prior art to ensure such correct tracking. One early method, used in disk drives, was to use a dedicated disk in a multi-disk pack which contained specialized servo coding to provide a clear indication of head location on the servo control disk. The dedicated disk was rigidly linked to the remaining disks in the pack, thereby ensuring precise knowledge of the position of the heads relative to the track. Unfortunately, this approach requires the loss of significant disk surface to the dedicated servo information, surface which could otherwise be used for data storage. In addition, this technique requires the added complexity of a dedicated servo disk head rigidly connected to the read/write head.
An approach currently used to ensure correct tracking in disk drives is to embed servo information onto the data tracks themselves. These "burst" servo approaches require that easily distinguished, often intense, servo bursts be placed onto specific angular positions around the track. In this manner, a servo control system can readily determine the exact location of the head on the track, without the need to decode the data signals themselves, since the bursts are both intense and distinctive. More sophisticated versions of this approach use multiple burst segments covering parts of multiple, adjacent tracks, staggered both longitudinally and laterally along the tracks to enable exact determination of the head location.
Again, this approach requires that a significant amount of media be dedicated to servo information, surface area which could, otherwise, be used for data storage, thereby increasing the data storage capacity of the disk. Further, it may require that the head read multiple tracks. While this may be unavoidable for initially locating the correct track during relocation of the head from one data track to another, it may undesirably complicate the servo process during intra-track processes.
Other approaches used to ensure correct tracking have focused on utilizing measured signal strength to ensure that the head is correctly centered on the track. For example, read heads have been used having dual head elements, each generating a signal. If such a head were tracking correctly, these two signals should be substantially identical. If the head were to drift in one direction, the signal from the head element which is drifting away from the track would decrease. This signal behavior is used to track the head's centering on the track. The two signals from the head elements are summed to yield the data signal and subtracted to yield an error signal. The polarity of the error signal indicates the direction of transverse drift, and the intensity of the error signal indicates the severity of drift.
While such an approach is helpful in ensuring centering of the head on the track, in practice this approach requires separate read and write heads, with a single-element write head. For example, problems may arise if playback by such a bifurcated head were attempted of a track recorded by a different bifurcated head since minuscule differences in alignment of the head elements of the two different heads could lead to mistracking in the read mode. In such a case, recording would have to be by a single element write head to ensure uniformity of the signal laterally across the track.
Hence, a practical implementation of such a system would require separate read and write heads. However, the use of such separate heads in a disk drive would undesirably increase the cost and complexity of the drive, and, hence, this method is of limited value in actual computer disk drives.
As a result of these problems, it would be desirable to provide a tracking servo system for disk drives, tape recorders, etc., utilizing a single read/write assembly, where tracking is based on the signal from an actual single track with a minimum of dedicated servo control space on the media. Such a system should have a high bandwidth and be of practical use in repeatedly accessing the same track at a desired location.